The invention relates to a hydraulic system for a power-take-off clutch in a utility vehicle such as an agricultural tractor.
As a rule, power-take-offs in utility vehicles such as agricultural tractors, for example, are engaged for only some of the time that the vehicle is used. When engaged, the clutch has to transmit in large torque to the power-take-off to drive an implement of some sort. Furthermore, substantial quantities of oil have to be delivered to the lubricating circuit in order to lubricate and cool the power-take-off clutch and gearbox. For this reason, the powertake-off hydraulic system is designed to operate at a relatively high system pressure. When the power-take-off clutch is released, only the power-take-off brake, which is subject to minimal loading, is engaged and only the part of the released clutch on the drive side is supplied with lubricating oil. A significantly lower system pressure would be sufficient for this purpose. Since the power requirement of the pump depends on the system pressure, the pump will undergo a considerable loss of power if the system pressure remains constant, which in turn causes the hydraulic medium to heat up considerably.
A hydraulic system has already been disclosed which provides a means of keeping the loss of power in the pump as low as possible, in which a high system pressure is applied to the power-take-off (PTO) clutch and a comparatively reduced system pressure is applied to the PTO brake. The known hydraulic system contains a control valve for selecting the clutch or the brake and two pressure-limiting valves, the first of these pressure-limiting valves limiting the pump pressure to the high system pressure as soon as the clutch is selected via the control valve and the second pressure-limiting valve limiting the pump pressure to the lower system pressure when the brake is engaged as soon as the brake is selected via the control valve. Hydraulic fluid flowing off through the pressure-limiting valves is fed into the lubricating circuit via respective check-valves. The disadvantage of the known system is the relatively high number of components it requires. In addition, the control valve, a 4/2 way valve, is restricted to use on hydraulic systems for PTOs with alternately selected clutch and brake.